Acid gas removal from various gas streams, and especially removal of carbon dioxide from natural gas streams has become an increasingly important process as the acid gas content of various gas streams increases. For example, the carbon dioxide concentration in natural gas from enhanced oil recovery will typically increase from 10% to about 60%. There are numerous processes for acid gas removal known in the art, and all or almost all of those may be categorized into one of three categories.
In the first category, one or more membranes are used to physically separate the acid gas from a gaseous feed stream. A typical membrane system includes a pre-treatment skid and a series of membrane modules. Membrane systems are often highly adaptable to accommodate treatment of various gas volumes and product-gas specifications. Moreover, membrane systems are relatively compact, therefore rendering membrane systems an especially viable option for offshore gas treatment. However, membrane systems are susceptible to deterioration from heavy hydrocarbons content in the feed gas. Moreover, carbon dioxide removal to relatively low carbon dioxide content typically requires multiple stages of membrane separators and recompression between stages, which tend to be relatively expensive.
In the second category, a chemical solvent is employed that reacts with the acid gas to form a (typically non-covalent) complex with the acid gas. In processes involving a chemical reaction between the acid gas and the solvent, the crude gases are typically scrubbed with an alkaline salt solution of a weak inorganic acid as, for example, described in U.S. Pat. No. 3,563,695, or with alkaline solutions of organic acids or bases as, for example, described in U.S. Pat. No. 2,177,068. Such chemical reaction processes generally require heat regeneration and cooling of the chemical solvents, and often involve recirculation of large amounts of chemical solvent. Moreover, the quantity of chemical solvent required to absorb increasing amounts of acid gases generally increases significantly, thus making use of chemical solvents problematic where the acid gas content increases over time in the feed gas.
In the third category, a physical solvent is employed for removal of acid gas from a feed gas, wherein the acid gas does react in an appreciable amount with the solvent. The physical absorption of the acid gas predominantly depends upon use of solvents having selective solubilities for the particular acid gas (e.g., CO2 or H2S) gaseous components to be removed and is further dependent upon pressure and temperature of the solvent. For example, methanol may be employed as a low-boiling organic physical solvent, as exemplified in U.S. Pat. No. 2,863,527. However, the energy input requirements for cooling are relatively high, and the process generally exhibits greater than desired methane and ethane absorption, thereby necessitating large energy inputs for recompression and recovery.
Alternatively, physical solvents may be operated at ambient or slightly below ambient temperatures, including propylene carbonates as described in U.S. Pat. No. 2,926,751 and those using N-methylpyrrolidone or glycol ethers as described in U.S. Pat. No. 3,505,784. While such solvents may advantageously reduce cooling requirements, most propylene carbonate-based absorption processes are limited to absorption pressures of less than 1000 psi (i.e., at sub-critical pressure). In further known methods, physical solvents may also include ethers of polyglycols, and specifically dimethoxytetraethylene glycol as shown in U.S. Pat. No. 2,649,166, or N-substituted morpholine as described in U.S. Pat. No. 3,773,896. While use of physical solvents avoids at least some of the problems associated with chemical solvents and/membranes, various new difficulties arise. Among other things, carbon dioxide removal from high pressure feed gases is often limited to sub-critical pressures. Furthermore, as the water content increases, freezing may occur in the solvent circuit, thus necessitating a relatively high temperature and thereby reducing the efficiency of the absorption process. In another aspect, physical solvent generally requires steam or external heat for solvent regeneration in order to produce a very lean solvent suitable for removal of acid gas to the ppm level.
Thus, although various configurations and methods are known to remove acid gases from a feed gas, all or almost all of them suffer from one or more disadvantages. Therefore, there is still a need to provide methods and configurations for improved acid gas removal.